FIG. 23 shows appearance of an ordinary code division multiplexing transmission system. The system will be briefly described below.
Before describing the system, code division multiplexing transmission will be briefly described. Multi code direct sequence spread spectrum modulation (MC-DSSS) is a technique for dividing an information signal to be transmitted into blocks in units of a plurality of bits, spreading the bits in one block by orthogonal spreading codes, respectively, multiplexing the resultant spread sequences on a time axis and transmitting the multiplexed signal. A spreading processing on each bit is also regarded as block coding. Therefore, in the following description, each spreading code in the MC-DSSS will be referred to as “time block code”. Further, a modulation technique for multiplying an information signal by a plurality of time block codes, parallel-multiplexing resultant signals and transmitting the parallel-multiplexed signal will be generically referred to as “code division multiplexing modulation” similarly to the MC-DSSS modulation.
Referring to FIG. 23, a code division multiplexing transmission system 100 includes a transmitter 100a and a receiver 100b. A signal transmitted from the transmitter 100a is received by the receiver 100b via a channel 109. An information signal S[n] is modulated by a modulation processing unit 101. A resultant modulated signal x(t) is processed by an up-converter 103, subjected to a radio frequency (high frequency) processing by an RF 105 and transmitted from a transmit antenna 107. The transmitted signal is received by a reception antenna 111 via the channel 109. The received signal is subjected to a low noise amplifier and a filter processing by a LNA&reception filter 113 and processed by a down-converter 115. A received signal r[t] is processed by a demodulation processing unit 117 and a decoded signal S^[n] is obtained.
Meanwhile, in a direct sequence-code division multiple access (DS-CDMA), different user signals are multiplied by a plurality of spreading codes, respectively. Further, a receiver side performs multipath separation on the resultant signals using autocorrelation characteristic of each spreading code exhibiting a sharp peak and maximum-ratio combines the signals, thereby separating the user signals multiplexed on a time axis while obtaining path diversity effect. Orthogonal codes such as Walsh codes are used as the spreading code on a DS-CDMA downlink. Even if orthogonal codes are multiplexed and transmitted from a transmitter side, orthogonality is eliminated on the receiver side due to distortion of the channel. However, downlink signals multiplexed by a plurality of users received by respective terminals are transmitted via the same channel on a downlink. Due to this, the orthogonality among the spreading codes can be reproduced by performing chip level equalization for suppressing the distortion of the channel (Non-Patent Document 1 and Non-Patent Document 2).
FIG. 24 shows a configuration of the modulation processing unit 101 shown in FIG. 23 if OFDM as a conventional technique is applied to the unit 101.
In FIG. 24, the modulation processing unit 101 includes an S/P conversion unit 1011, an MOD_OFDM unit 1013, a cyclic prefix addition unit 1015 and a waveform shaping filter 1017. Each of information signals S[i] obtained by conversion performed by the S/P conversion unit 1011 are multiplied by a modulation code C_i[n] by a multiplication unit 1014a of the MOD_OFDM unit 1013. It is assumed herein that a length of the modulation code C_i[n] is N. Sampling sequences are often arranged in elements of vectors in order of time, respectively to be expressed as a multidimensional vector as follows. For example, (C_i[0], C_i[1], . . . , C_i[N−1]) are expressed as an N-order code vector. In the OFDM, sinusoidal waves different in frequency and orthogonal to one another are used as C_i[n]. A summation unit 1014b of the MOD_OFDM unit 1013 calculates a sum to obtain X. The cyclic prefix addition unit 1015 performs a processing for adding a cyclic prefix at a length of G to this X to obtain XT. Insertion of the cyclic prefix means addition of a copy of a suffix G symbol of X[n] to a prefix of X[n] without performing any processing. Further, the waveform shaping filter 1017 performs the filter processing to obtain x(t) Furthermore, it is to be noted that the processing performed by the MOD_OFDM unit 1013 surrounded by a dotted line is realized by P/S-converting an output signal after IFFT in the actual OFDM.
FIG. 25 shows a configuration of the demodulation processing unit 117 shown in FIG. 23 if the OFDM as the conventional technique is applied to the unit 117.
The demodulation processing unit 117 includes an M symbol sampling unit 1171, a cyclic prefix removal unit 1173, a filter 1175 and a DMOD_OFDM unit 1177. M is set to N+G, i.e., M=N+G and the M symbol sampling unit 1171 performs a sampling processing on r(t) to obtain R′. Further, the cyclic prefix removal unit 1173 removes the cyclic prefix inserted on the transmitter side and, therefore, processes R[n] in units of blocks each having a length of N. Thereafter, the filter 1175 makes a phase correction of each sub-carrier of the R using P in the form of a complex matrix of N rows by N columns. The DMOD_OFDM unit 1177, which functions as an inner product unit, performs an inner product operation between obtained R_f and C_i to obtain S^[n]. In this case, when two row vectors A and B equal in number of orders are given, (A, B) represents an inner product operation and (A, B)=AH·B, where AH represents a complex transposed matrix of the matrix A and is a matrix normally referred to as “complex conjugate transpose matrix of A”. In the actual OFDM, the DMOD_OFDM unit 1177 surrounded by a dotted line is realized by using FFT after S/P-converting R_f (n).
The above-stated OFDM modulation has been recently frequently adopted in a broadband wireless communication system. The OFDM modulation is regarded as a type of code division multiplexing modulation using time block codes (sinusoidal codes) orthogonal to one another. During demodulation, the orthogonality among the codes is lost by the distortion of the channel as stated. However, by providing a guard interval (GI), filling up Cyclic Prefix (CP) signals, and receiver side' s extracting intervals without adjacent block interference, the orthogonality among the sinusoidal wave codes is maintained and the adjacent block interference is eliminated even on the distorted channel. If a GI length is sufficiently large due to spread of a delay of the channel, each of the received sinusoidal wave codes after elimination of the GI subjected to DFT has only one frequency component without overlap. During demodulation, only by making phase adjustment of the respective sub-carriers on a frequency axis, it is possible to obtain a function corresponding to distortionless transmission even on a frequency selective channel. Non-Patent Documents 3 and 4 pay an attention to this feature of the OFDM and propose techniques for inserting CP into code division multiplexing modulation using such time block codes such as Walsh codes used in the CDMA and for performing FFT processing on a receiver side to equalize the signals on a frequency axis. According to the Non-Patent Documents 3 and 4, an MMSE index is adopted for setting of an equalization weight, thereby obtaining characteristics equal to or higher than the chip level equalization and RAKE reception described in the Non-Patent Documents 1 and 2.
[Non-Patent Document 1]
I. Ghauri and D. Slock, “Linear receivers for DS-CDMA downlink exploiting orthogonality of spreading sequences”, Proc. Asilomar Conference on Signals, Systems and Computers, Vol. 1, pp. 650-654, November 1998.
[Non-Patent Document 2]
K. Hooli, M. Latva-aho and M. Juntti, “Multiple access interference suppression in CDMA with linear chip equalizers in WCDMA downlink receivers,” Proc. GLOBECOM 99, Vol. General Conference (Part A), pp. 467-471, December 1999.
[Non-Patent Document 3]
F. Adachi, T. Sato and T. Itagaki, “Performance of multicode DS-CDMA using frequency domain equalization in a frequency selective fading channel”, Electronics Letters, vol. 39, No. 2, pp. 239-241, January 2003.
[Non-Patent Document 4]
F. Adachi, K. Takeda and H. Tomeba, “Frequency-Domain Pre-Equalization for Multicode Direct Sequence Spread Spectrum Signal Transmission,” IEICE Trans. Comm., Vol. E88-B, No. 7, pp. 3078-3081, July 2005.